A major cell surface glycoprotein (CSP) that is involved in cellular adhesion is generally decreased after cells are established as permanent cell lines, and is depleted further after malignant transformation of fibroblasts in vitro. CSP's overall protein structure has been determined. It consists of disulfide-linked dimers and polymers of a 220,000 dalton subunit having a highly asymmetric shape. CSP's oligosaccharides are attached via asparaginyl linkages. Inhibition of this glycosylation using the new drug tunicamycin does not alter its synthesis or secretion, but results in a marked susceptibility of CSP to proteolytic degradation. Immunofluorescence staining using affinity-purified antibody to CSP localizes it to relatively immobile fibrillar meshworks under and between cells. Treatment of cells with anti-CSP results in a morphology similar to that of many transformed cells, and a gradual antibody-induced redistribution of antigen on the cell surface. Our objectives will be to determine the composition and structure of CSP's active site(s), and the mechanisms by which it helps to maintain normal cell behavior.